Helmut Suess

Microwave imaging of the brightness temperature distribution of extended areas in the near and far field using two‐dimensional aperture synthesis with high spatial resolution

Aperture synthesis is an attractive alternative to conventional passive microwave imaging systems for remote sensing applications. Whenever a high spatial resolution is desired or the imaging process should work like an optical camera or when microwave remote sensing in the near field has to be distance adaptive, conventional systems cause a lot of problems. However, aperture synthesis for practical applications requires a lot of single receivers and correlators. This can cause other problems because of the large number of possible imperfections. To get an estimation of such effects, the imaging expressions for the aperture synthesis principle are developed for the near- and far-field conditions of a real system. To support the theory and to get an estimation of the feasibility of this imaging method, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown. For the near-field case a special reconstruction algorithm was developed and is described theoretically and validated by the experiments. A discussion of the temperature resolution of aperture synthesis concludes the paper.

3D Tower-Turntable ISAR Imaging

This paper studies the effects of peculiarities of an experimental stepped-frequency measurement system in a tower-turntable setup on three-dimensional inverse synthetic aperture radar imaging by simulation of ideal point scatterers. Particular a phase drift, an offset of the elevation rotation center and near range distortion have to be compensated. Furthermore, first imaging results of measured 3D data will be presented.

Fully polarimetric measurements of brightness temperature distributions with a quasi-optical radiometer system at 90 GHz

The thermal microwave radiation of our natural and man-made environment contains many objects with fully polarimetric information. Based on this assumption a quasi-optical imaging radiometer system was designed and realized for the determination of the complete Stokes vector. To demonstrate the system performance, the beam quality was verified by measurements and a polarimetric calibration procedure was developed. Measurements on selected objects have been carried out to demonstrate the polarimetric effects primarily of the third and fourth component of the Stokes vector. The measured results indicate new possible applications for remote sensing and material testing.

Fast ISAR image generation through localization of persistent scattering centers

Imagery data acquired by recently launched space borne SAR systems demonstrate a very good spatial resolution (e.g. one meter with TerraSAR-X). The designs of such complex systems make it compulsory to do SAR end-to-end simulations to optimize image quality (e.g. spatial and radiometric resolution, ambiguity suppression, dynamic range, etc.). The most complex, critical and challenging modules have to be designed for the generation of SAR raw data and SAR image generation, because the limits of computability and memory requirements are reached very quickly. Moreover, the analysis of SAR images is a demanding task, because of their sensor specific effects. Therefore, a simulation tool is under development to analyze realistic target features and make the scattering processes transparent to the user. With the method presented in this paper, SAR images of complex scattering bodies can be generated in a very efficient way. This is done by directly localizing scattering centers and identifying their persistency along the synthetic aperture. Thus the usual raw data generation and processing steps are dropped. The resulting images show a very good similarity to reality, because scattering centers due to multipath propagation effects are also handled. Furthermore this toolkit makes it possible to visualize the scattering centers and their evolution, by mapping them on the 3D structure of the scattering body. This results in transparency of the whole scattering process, which greatly improves the understanding of the image effects. The paper presents this new approach for the application of inverse SAR (ISAR) and first simulation results.

Passive microwave remote sensing for security applications

The security of persons or sensitive infrastructures is of increasing importance. Passive microwave remote sensing allows a daytime-independent non-destructive observation and examination of the objects of interest without artificial exposure under nearly all weather conditions. The penetration capability of microwaves enables the detection of hidden objects. Examples for various imaging experiments are shown. The experimental systems are used to investigate basic key parameters for the specific applications like suitable frequency band, required spatial resolution, sensitivity, and field of view. Systems close to real-time are under investigation and development.

Applications of simulation techniques for high resolution SAR systems

The interpretation of radar imagery can often be very challenging due to specific imaging effects. Additionally, a fusion with other remote sensing data is not easy to accomplish, because of the imaging radars slant range coordinate system. Simulation techniques are able to provide essential assistance or even practical solutions to such challenges. This paper shows the usage of a developed simulator in practical applications for signature analysis, data fusion and acquisition planning of spaceborne radar systems, but also for parametric end-to-end simulations of a very high resolution ground based radar.

Transforming optical image data into a SAR system's range-based image space

The fusion of image data from different sensor types is an important processing step for many remote sensing applications to maximize information retrieval from a given area of interest. The basic process to fuse image data is to select a common coordinate system and resample the data to this new image space. Usually, this is done by orthorectifying all those different image spaces, which means a transformation of the image’s projection plane to a geographic coordinate system. Unfortunately, the resampling of the slant-range based image space of a space borne synthetic aperture radar (SAR) to such a coordinate system strongly distorts its content and therefore reduces the amount of extractable information. The understanding of the complex signatures, which are already hard to interpret in the original data, even gets worse. To preserve maximum information extraction, this paper shows an approach to transform optical images into the radar image space. This can be accomplished by using an optical image along with a digital elevation model and project it to the same slant-range image plane as the one from the radar image acquisition. This whole process will be shown in detail for practical examples.

Depth-of-focus issues on spaceborne very high resolution SAR

Higher resolutions mean a more complex challenge on the SAR imaging not only in dealing with an at least quadratically growing data amount, but the more with stronger conditions on the focusing of the SAR image. In this study the defocusing effect is evaluated quantitatively by measuring the resulting resolution of a simulated ideal point scatterer depending on its height distance to the focusing plane. The simulations indicate a linear dependence of the depth-of-focus on the square of the nominal cross range resolution.

DLR activities on aperture synthesis radiometry

Imaging microwave radiometer systems in Earth observation from long distances are forced to use very large antenna apertures for an adequate spatial resolution following the wavelength over diameter law. In many cases the classical imaging principle of a linescanner cannot be applied because of the required movement of the antenna beam within the desired field of view (FOV). An alternative method is given by the principle of aperture synthesis from radio astronomy having the following main advantages: i) simultaneous acquisition of all image points, i.e. quasi real-time operation is possible (snapshot), ii) a wide FOV up to a complete hemisphere can be imaged, iii) available platform structures can be used for the creation of large apertures. In the ideal case the imaging operation is carried out in the spatial Fourier space, given by the baseline components or antenna distances u and v in wavelengths, using two-by-two interferometric techniques (correlation) on a thinned array of spatially distributed antennas. The spatial brightness temperature distribution T/sub B/(l,m) to be determined, given in direction cosines l and m, is calculated by an inverse Fourier transform (FT) of the measured complex and Hermitian visibility function V(u,v). This paper describes the CLEAN algorithm.

Airborne and groundbased passive millimeter-wave imaging at 37 and 90 GHz

Passive microwave imaging with conventional linescanner systems is an extensively proofed technique with a long tradition and experience in civil and military application fields. During the last couple of years another promising technique, the aperture synthesis method, has become more of interest because of the principal possibility to generate 2D images without moving the aperture. In the first past of this paper, representative measurement results are shown from a 90 GHz linescanner system, cooled with liquid nitrogen, with a spatial and radiometric resolution of 1 degree(s) and 1.7 K for flight measurements, respectively. In the second part, a groundbased aperture synthesis radiometer imaging system at 37 GHz is described. Basically the system consists of a two-element interferometer with a variable baseline, which enables the complete sampling of the uv- plane sequentially. As a consequence this imaging equipment is only suited for the mapping of stationary targets. Experimental measurement results are demonstrated which were acquired in the near and far field with a spatial resolution of 0.6 degree(s) and a temperature resolution of about 1.5 K.